Recently, the use of waste energy has become a focus of attention, because the following problems have been in progress: the depletion of fossil energy resources, global warming, and air pollution. Since thermoelectric materials can be used for converting waste heat into electrical energy, electrical energy can be efficiently generated with conventional electric power-generating systems including such thermoelectric materials. Therefore, it is expected that new thermoelectric materials having higher efficiency are developed. Since a junction is cooled by the Peltier effect when a current is applied to a thermoelectric material, such a material is used for cooling systems. Therefore, demands have been made on new thermoelectric materials having higher efficiency.
Furthermore, the following new semiconductors have been recently developed at a feverish pace: wide-gap semiconductors, transparent semiconductors, and organic semiconductors. Carrier control is the key to use such semiconductors for devices. Various devices such as transistors, FETs, diodes, and solar cells, laser oscillators can be manufactured using, for example, junctions (pn junctions) consisting of p-type semiconductors, in which the major carriers are holes, and n-type semiconductors, in which the major carriers are electrons. Therefore, the carrier type is particularly a critical property parameter.
Carrier control is usually performed in such a manner that an undoped sample is subjected to impurity substitution or is heat-treated in an impurity gas atmosphere. However, since the dose is very small, the control is not easy and therefore such treatment is performed by trial and error in many cases. In particular, when the wide-gap semiconductors are treated, either a p type or an n type is apt to be obtained. Therefore, only either type has been fabricated and the conditions of fabrication are strictly limited. Thus, it is necessary to investigate more precise growing conditions and doping conditions.